Methods for the measurement and description of the flow of fluids (both liquids and gases) may be divided into two main classes: invasive techniques and non-invasive techniques. Invasive methods make use of devices such as probes, the introduction of markers, the measurement of pressure changes across restrictions, etc. The non-invasive methods make use of the external measurement of some flow-dependent property such as those relating to changes in optical properties, acoustic properties, electro-magnetic properties, etc. At the present time, several optical methods exist for gaining information about fluid flows. Laser doppler, schlerean optics, interferometic, holographic, etc., methods are know to the art. These methods require that the fluid be nearly transparent to the incident light. In addition to near perfect fluid transparency, doppler methods require the presence of scattering centers or particles. Natural or artificial dust particles or fluctuations in density can serve this purpose. Low scattering center concentration provides longer ranges but sensitivity suffers. Higher scattering center concentration provides higher sensitivity but limits the range (depth) of measurement. The use of higher transmit power levels increases range and provides a stronger scattered signal but power is usually restricted by practical upper limits. Schlerean, interferometric, and holographic methods work well in general only for gas flows where density variations may be larger than for liquids.
Acoustic methods have also been proposed and applied which make use of either the doppler scattered sound or transmitted sound along a single beam. The use of pulsed doppler methods allows measurement of the component of fluid velocity (not the true fluid veloity) along the acoustic beam. The same trade off between sensitivity, which govern laser doppler measurements also exist for acoustic doppler techniques.
The average fluid velocity component along a transmitted beam can be determined from the measured time of propagation along the beam. Flow meters designed around this principle are well known in the art.
All of these methods suffer from several common weaknesses: first, only the component of flow parallel to the beam is determined; second, only that region of the flow traversed by the beam is sampled. Thus, none of the previously mentioned methods in their simplest form measure true three-dimensional flow.